Composite particles for an electrode comprising lithium vanadyl phosphate (LiVOPO4), production process thereof and electrochemical device

ABSTRACT

Composite particles for an electrode comprising LiVOPO 4  particles and a metal, wherein the metal is supported on at least a portion of the surface of the LiVOPO 4  particles to form a metal coating layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to composite particles for an electrode, aproduction process thereof and an electrochemical device.

2. Related Background Art

Known examples of cathode active materials of lithium ion secondarybatteries include layered oxides (such as LiCoO₂, LiNiO₂ or LiNiMnCoO₂),spinel structure compounds (such as LiMn₂O₄) and lithium-containingphosphates (such as LiFePO₄).

Among these, although layered oxides allow the obtaining of highcapacity (for example, 150 mAh/g or more), they have the problems of lowthermal stability when highly charged and a lack of safety (overchargestability). In addition, although the spinel compound LiMn₂O₄(theoretical capacity: 148 mAh/g) has a stable structure and a highdegree of safety (overcharge stability), it easily elutes Mn³⁺ ions athigh temperatures (such as 45° C. or higher), thereby resulting in theproblem of low stability of battery properties at high temperaturesaccompanying anode deterioration caused thereby. Consequently,lithium-containing phosphates are used in place of layered oxides andspinel structure compounds from the viewpoints of safety andhigh-temperature stability. Examples of batteries using alithium-containing phosphate for the cathode active material aredescribed in Japanese Patent Application Laid-open No. 2004-303527 andJapanese Patent No. 3484003.

However, although a typical example of a lithium-containing phosphate inthe form of LiFePO₄ (theoretical capacity: 169 mAh/g) has a high degreeof safety and high-temperature stability, the discharge voltage relativeto lithium is 3.3 to 3.4 V, which is lower than that of other cathodeactive materials. In addition, these materials are extremely sensitiveto the atmosphere (a reducing environment is required) and temperatureconditions during synthesis, and are disadvantageous for inexpensive,large-scale production.

On the other hand, another lithium-containing phosphate in the form ofLiVOPO₄ (theoretical capacity: 159 mAh/g) has a stable structure and adischarge voltage roughly equal to other cathode active materials (3.8to 3.9 V relative to lithium), but does not particularly require areducing atmosphere during synthesis as with LiFePO₄. However, itdemonstrates the characteristic problem of lithium-containing phosphatesof low electron conductivity, making it difficult to adequatelydemonstrate the properties thereof in electrode structures of the priorart in which it is simply mixed with a conductive auxiliary agent.

SUMMARY OF THE INVENTION

With the foregoing in view, an object of the present invention is toprovide composite particles for an electrode capable of forming anelectrochemical device having superior discharge voltage and dischargecapacity as well as superior rate characteristics by using as an activematerial, a production process thereof, and an electrochemical devicethat uses these composite particles for an electrode.

In order to achieve the above-mentioned object, the present inventionprovides composite particles for an electrode comprising LiVOPO₄particles and a metal, wherein the metal is supported on at least aportion of the surface of the LiVOPO₄ particles to form a metal coatinglayer.

These composite particles for an electrode allow the obtaining ofsuperior electron conductivity as a result of the surface of the LiVOPO₄particles being coated with the metal coating layer. In particular sincethe metal is not supported in the form of particles, but rather isformed as a layer, in comparison with the case of supporting metalparticles, in addition to being able to inhibit sloughing of the metal,since the surface of the LiVOPO₄ particles can be coated using a smalleramount and more efficiently than in the case of particle coating, thecontent of supported metal in the composite particles can be reducedwhile effectively imparting conductivity. Consequently, anelectrochemical device using these composite particles as an activematerial allows the obtaining of superior discharge voltage and superiordischarge capacity while also allowing the obtaining of superior ratecharacteristics.

In addition, when the length of the outer circumference of theabove-mentioned LiVOPO₄ particles in a cross-section thereof isdesignated as L, and the length of the portion of the outercircumference of the LiVOPO₄ particles where the metal coating layer isformed is designated as L′, then the coating ratio of the compositeparticles for an electrode of the present invention represented by(L′/L) is preferably 0.2 or more. As a result of making this coatingratio 0.2 or more, the electron conductivity of the composite particlescan be adequately enhanced, thereby making it possible to form anelectrochemical device having even better discharge capacity and ratecharacteristics.

In addition, the composite particles for an electrode of the presentinvention preferably have a BET specific surface area of 0.5 to 15.0m²/g. As a result of BET specific surface area being within the aboverange, an adequate electron conduction path can be formed when formingan electrode, and the amount of binder in the active material-containinglayer can be reduced.

In addition, in the composite particles for an electrode of the presentinvention, the metal preferably contains at least one type selected fromthe group consisting of Al, Au, Ag and Pt. As a result of using thismetal, adequate electron conductivity of the coating layer and stabilityin a cathode operating environment can be obtained.

In addition, in the composite particles for an electrode of the presentinvention, the thickness of the metal coating layer is preferably 10 to300 nm. As a result of the thickness of the metal coating layer beingwithin this range, the composite particles allow the obtaining ofadequate electron conductivity, thereby making it possible to morereliably form an electrochemical device having superior dischargevoltage and discharge capacity, and superior rate characteristics.

The present invention also provides an electrochemical device providedwith an electrode containing the composite particles for an electrode ofthe present invention. According to this electrochemical device, byusing an electrode containing the composite particles for an electrodeof the present invention demonstrating the previously described effects,in addition to obtaining superior discharge voltage and dischargecapacity, superior rate characteristics can also be obtained.

The present invention also provides a process for producing compositeparticles for an electrode which comprise LiVOPO₄ particles and a metal,the metal being supported on at least a portion of the surface of theLiVOPO₄ particles to form a metal coating layer, the process comprisinga fluidized layer formation step of forming the metal coating layer onat least a portion of the surface of the LiVOPO₄ particles byintroducing the LiVOPO₄ particles and metal particles into a fluidizedbed in which an air flow has been generated and forming a fluidizedlayer.

According to this production process, composite particles for anelectrode of the present invention that demonstrate the previouslydescribed effects can be produced efficiently and reliably.

Moreover, the present invention provides a process for producingcomposite particles for an electrode which comprise LiVOPO₄ particlesand a metal, the metal being supported on at least a portion of thesurface of the LiVOPO₄ particles to form a metal coating layer, theprocess comprising: a dispersion step of introducing the LiVOPO₄particles into a metal ion-containing solution to obtain an LiVOPO₄dispersion; and a reduction step of subjecting the LiVOPO₄ dispersion toreduction treatment.

According to this production process, composite particles for anelectrode of the present invention that demonstrate the previouslydescribed effects can be produced efficiently and reliably.

Here, the metal in the process for producing composite particles for anelectrode as described above preferably contains at least one typeselected from the group consisting of Al, Au, Ag and Pt. As a result ofusing this metal, adequate electron conductivity of the coating layerand stability in a cathode operating environment can be obtained.

According to the present invention, composite particles for an electrodecapable of forming an electrochemical device having superior dischargevoltage and discharge capacity as well as superior rate characteristics,a production process thereof and an electrochemical device that usesthese composite particles for an electrode can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an example of thebasic configuration of the composite particles for an electrode of thepresent invention;

FIG. 2 is a front view showing a preferred embodiment of anelectrochemical device of the present invention;

FIG. 3 is a developed view of the inside of the electrochemical deviceshown in FIG. 2 as viewed from the direction of a line normal to thesurface of an anode 10;

FIG. 4 is a schematic cross-sectional view obtained by cutting theelectrochemical device shown in FIG. 2, along line X1-X1 in FIG. 2;

FIG. 5 is a schematic cross-sectional view of the major part obtained bycutting the electrochemical device shown in FIG. 2, along line X2-X2 inFIG. 2;

FIG. 6 is a schematic cross-sectional view of the major part obtained bycutting the electrochemical device shown in FIG. 2, along line Y-Y inFIG. 2;

FIG. 7 is a schematic cross-sectional view showing an example of thebasic configuration of a film serving as a constituent material of acase of the electrochemical device shown in FIG. 2;

FIG. 8 is a schematic cross-sectional view showing another example ofthe basic configuration of a film serving as a constituent material of acase of the electrochemical device shown in FIG. 2;

FIG. 9 is a schematic cross-sectional view showing an example of thebasic configuration of an anode of the electrochemical device shown inFIG. 2; and

FIG. 10 is a schematic cross-sectional view showing an example of thebasic configuration of a cathode of the electrochemical device shown inFIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following provides a detailed explanation of preferred embodimentsof the present invention with reference to the drawings. Furthermore, inthe drawings, the same reference symbols are used to indicate the sameor equivalent components, and duplicative explanations are omitted.Positional relationships such as up, down, left and right are based onthe positional relationships shown in the drawings unless specificallyindicated otherwise. Moreover, the dimensional ratios in the drawingsare not limited to the ratios shown in the drawings.

First, an explanation is provided of the composite particles for anelectrode of the present invention along with a production processthereof. FIG. 1 is a schematic cross-sectional view showing the basicconfiguration of a preferred embodiment of the composite particles foran electrode of the present invention. As shown in FIG. 1, a compositeparticle for an electrode 8 is composed of an electrode active materialin the form of an LiVOPO₄ particle 4, and a metal coating layer 6composed of a metal formed on at least a portion of the surface of theLiVOPO₄ particle 4.

When the length of the outer circumference of the LiVOPO₄ particle 4 ina cross-section thereof as shown in FIG. 1 is designated as L, and thelength of the portion of the outer circumference of the LiVOPO₄ particle4 where the metal coating layer 6 is formed is designated as L′(sameunits as L), then the coating ratio of the composite particles 8represented by (L′/L) is preferably 0.2 or more. Furthermore, in thecase the metal coating layer 6 is formed at a plurality of locations ina cross-section thereof as in the composite particle 8 shown in FIG. 1,L′ is the total value of the lengths of all portions where the metalcoating layer 6 is formed on the outer circumference of the LiVOPO₄particle 4.

In addition, although it is preferable that the above-mentioned coatingratio be 0.2 or more, it is more preferably 0.3 or more, even morepreferably 0.5 or more, and particularly preferably 0.6 to 0.9. In thecase this coating ratio is less than 0.2, the coating state of theLiVOPO₄ particles by the metal coating layer becomes inadequate ascompared with the case of the coating ratio being 0.2 or more, therebyresulting in a tendency for the electron conductivity of the compositeparticles to decrease.

In addition, the BET specific surface area of the composite particle 8is preferably 0.5 to 15.0 m²/g, and more preferably 0.6 to 10.0 mg²/g.If the BET specific surface area is less than 0.5 m²/g, the metalcoating of the composite particle 8 tends to be inadequate, while if theBET specific surface area exceeds 15.0 m²/g, a large amount of binder isrequired when producing an electrode coating using this compositeparticle 8, the ratio of active material in the electrode decreases, andit tends to become difficult to express high capacity as an electrode.

In addition, the thickness of the metal coating layer 6 in the compositeparticle 8 is preferably 10 to 300 nm and more preferably 20 to 200 nm.If the thickness of the metal coating layer 6 is less than 10 nm, theelectron conductivity of the composite particle 8 tends to beinadequate, while if the thickness exceeds 300 nm, the electricalcapacity per unit mass of the composite particle 8 tends to decrease.

Examples of the metal that composes the metal coating layer 6 in thecomposite particle 8 include Al, Au, Ag and Pt. These metals arepreferable since they are able to further improve the electronconductivity of the composite particle 8 and are stable in a cathodeoperating environment.

In addition, the metal content of the composite particle 8 based on thetotal mass of said composite particle 8 (content of metal supported onLiVOPO₄ particles) is preferably 0.5 to 6.0% by mass, more preferably1.0 to 6.0% by mass, and particularly preferably 2.0 to 5.0% by mass. Ifthe metal content is less than 0.5% by mass, the electron conductivityof the composite particle 8 tends to be inadequate, while if the metalcontent exceeds 6.0% by mass, the amount of metal in the compositeparticle 8 becomes unnecessarily large, and tends to lead to a decreasein electrode capacity due to a decrease in the proportion of activematerial.

The composite particles for an electrode of the present invention asdescribed above can be produced by, for example, the production processdescribed below. The following provides an explanation of a first andsecond production process for producing the composite particles for anelectrode of the present invention.

A first production process of the composite particles for an electrodeof the present invention is a process for physically forming a metalcoating layer on the surface of LiVOPO₄ particles. Namely, the firstproduction process is a process comprising a fluidized layer formationstep in which a metal coating layer is formed on at least a portion ofthe surface of LiVOPO₄ particles by introducing LiVOPO₄ particles andmetal particles into a fluidized bed in which an air flow has beengenerated to form a fluidized layer.

LiVOPO₄ particles can be obtained by, for example, mixing a Li source, Vsource and PO₄ source at the stoichiometric ratio of LiVOPO₄ followed byfiring at 450 to 600° C. Examples of Li sources include Li₂CO₃, LiOH andlithium acetate. Examples of V sources include V₂O₅ and NH₂VO₃. Examplesof PO₄ sources include NH₄H₂PO₄ and (NH₄)₂HPO₄. Since LiVOPO₄ particlesobtained in this manner have an orthorhombic crystal structure and havebetter symmetry than triclinic structures obtained at highertemperatures, high Li ion insertion and elimination capacities can berealized.

In addition, examples of metal particles include particles composed ofAl, Au, Ag and Pt. These metals are preferable since they are able tofurther improve the electron conductivity of the compound particle 8 andare stable in a cathode operating environment.

Here, the ratio of the average particle diameter of the LiVOPO₄particles to the average particle diameter of the metal particles ispreferably 10:1 to 100:1, and more preferably 20:1 to 100:1. As a resultof making this ratio of average particle diameters within the aboveranges, coating of subparticles in the form of metal onto core particlesin the form of LiVOPO₄ particles proceeds selectively, and aggregationbetween subparticles can be inhibited.

Furthermore, the specific average particle diameter of the LiVOPO₄particles is preferably 0.2 to 10 μm and more preferably 0.2 to 6 μm. Onthe other hand, the specific average particle diameter of the metalparticles is preferably 10 to 200 nm and more preferably 10 to 150 nm.

In the fluidized layer formation step, the LiVOPO₄ particles and metalparticles are introduced into a fluidized layer rotating at high speed(preferably 13000 to 20000 rpm) and mixed. As a result, the metalparticles collide at high speed with the LiVOPO₄ particles, and therelatively soft metal particles are deformed while being coated onto thesurface of the LiVOPO₄ particles, resulting in the formation of a metalcoating layer.

In addition, in the fluidized layer formation step, mixing of theLiVOPO₄ particles and metal particles is preferably carried out in aninert atmosphere such as Ar, He or N₂ from the viewpoint of reactivityof the metal fine particles.

Since a metal coating layer obtained in this manner is physically firmlyadhered to the surface of the LiVOPO₄ particles, in addition to it beingdifficult for the metal coating layer to separate from the LiVOPO₄particles, the resulting layer has a suitable thickness as describedabove (for example, 20 to 300 nm).

A second production process of the composite particles for an electrodeof the present invention is a process for chemically forming a metalcoating layer on the surface of LiVOPO₄ particles. Namely, this secondproduction process is a process comprising a dispersion step, in whichLiVOPO₄ particles are introduced into a metal ion-containing solution toobtain an LiVOPO₄ dispersion, and a reduction step, in which the LiVOPO₄dispersion is subjected to reduction treatment. LiVOPO₄ particles can beobtained according to the method explained in the first productionprocess described above.

A solution containing ions such as Al, Au, Ag or Pt ions can be used forthe metal ion-containing solution. In the case of using these metalion-containing solutions, the electron conductivity of the resultingcomposite particle 8 can be further improved and the composite particle8 is stable in a cathode operating environment, thereby making thispreferable.

The metal ion-containing solution can be obtained by dissolving a metalsalt and the like serving as a supply source of the metal ions, such asgold chloride (HAuCl₄) or platinum chloride (H₂PtCl₆), in a solvent.

In addition, an example of a solvent composing the metal ion-containingsolution is water.

In the above-mentioned dispersion step, the LiVOPO₄ particles areintroduced and dispersed in the metal ion-containing solution to obtainan LiVOPO₄ dispersion.

Next, in the above-mentioned reduction step, the resulting LiVOPO₄dispersion is reduced. In this reduction step, the LiVOPO₄ dispersion isfirst heated to remove the solvent followed by the formation of anLiVOPO₄/metal (Me) precursor and further heating (firing) this precursorin an Ar, H₂ or N₂ atmosphere (normal pressure), reducing gas atmosphere(reduced pressure) or an air atmosphere to precipitate metal on thesurface of the LiVOPO₄ particles and form a metal coating layer. Here,the firing temperature of the precursor is preferably 150 to 400° C. andmore preferably 150 to 300° C.

The metal content of composite particles obtained according to the firstand second production processes (content of metal supported on LiVOPO₄particles) based on the total mass of the composite particles ispreferably 0.5 to 6.0% by mass, more preferably 1.0 to 6.0% by mass andparticularly preferably 2.0 to 5.0% by mass. If the metal content isless than 0.5% by mass, the metal coating layer is in the form ofislands and conductivity tends to decrease. On the other hand, if themetal content exceeds 6.0% by mass, the electrical capacity per unitmass of the composite particles tends to decrease. Furthermore, the formin which the metal coating layer is coated on the composite particles ispreferably such that the metal coating layer is coated in the form of athin film instead of islands around the periphery of the LiVOPO₄ parentparticles.

Furthermore, the composite particles for an electrode of the presentinvention can also be produced by a process other than the first andsecond production processes described above. Examples of processes forproducing composite particles other than the first and second productionprocesses described above include a process in which LiVOPO₄ particlesand metal particles are introduced into a dispersion medium and thenmixed with ball mill, and a process in which composite particles arechemically synthesized from a solution containing a Li source, V sourceand PO₄ source that compose LiVOPO₄ and metal ions.

Next, an explanation is provided of an electrochemical device of thepresent invention. An electrochemical device of the present invention isprovided with an electrode that contains the composite particles for anelectrode of the present invention as described above. Morespecifically, an electrochemical device of the present invention has aconfiguration that is provided with an anode, a cathode and anelectrolyte layer having ion conductivity, wherein the anode and thecathode are arranged in opposition to each other with the electrolytelayer there between, and at least one of the anode and the cathode is anelectrode containing the composite particles for an electrode of thepresent invention as described above. Furthermore, in the presentdescription, an “anode” refers to that which is a negative electrodebased on the polarity during discharge of the electrochemical device,while a “cathode” refers to that which is a positive electrode based onthe polarity during discharge of the electrochemical device.

FIG. 2 is a front view showing a preferred embodiment of anelectrochemical device of the present invention (lithium ion secondarybattery). In addition, FIG. 3 is a developed view of the inside of theelectrochemical device shown in FIG. 2 as viewed from the direction of aline normal to the surface of an anode 10. Moreover, FIG. 4 is aschematic cross-sectional view obtained by cutting the electrochemicaldevice shown in FIG. 2, along line X1-X1 in FIG. 2. In addition, FIG. 5is a schematic cross-sectional view of the major part obtained bycutting the electrochemical device shown in FIG. 2, along line X2-X2 inFIG. 2. In addition, FIG. 6 is a schematic cross-sectional view of themajor part obtained by cutting the electrochemical device shown in FIG.2, along line Y-Y in FIG. 2.

As shown in FIGS. 2 to 6, an electrochemical device 1 is mainly composedof a plate-like anode 10 and a plate-like cathode 20 in mutualopposition, a plate-like separator 40 arranged between and adjacent tothe anode 10 and the cathode 20, an electrolyte solution containinglithium ions (a non-aqueous electrolyte solution in the presentembodiment), a case 50 that houses these components in a sealed state,an anode lead 12 of which one end is electrically connected to the anode10 while the other end protrudes outside the case 50, and a cathode lead22 of which one end is electrically connected to the cathode 20 whilethe other end protrudes outside the case 50.

The following provides a detailed explanation of each constituentfeature of the present embodiment based on FIGS. 2 to 10.

First, an explanation is provided of the anode 10 and the cathode 20.FIG. 9 is a schematic cross-sectional view showing an example of thebasic configuration of the anode 10 of the electrochemical device 1shown in FIG. 2. In addition, FIG. 10 is a schematic cross-sectionalview showing an example of the basic configuration of the cathode 20 inthe electrochemical device 1 shown in FIG. 2.

The anode 10 shown in FIG. 9 is composed of a current collector 16 andan anode active material-containing layer 18 formed on the currentcollector 16. In addition, the cathode 20 shown in FIG. 10 is composedof a current collector 26 and a cathode active material-containing layer28 formed on the current collector 26.

At least one of the anode active material-containing layer 18 and thecathode active material-containing layer 28 contains the compositeparticles for an electrode of the present invention as described aboveas the active material. Furthermore, the composite particles for anelectrode of the present invention as described above effectivelyfunction as a cathode active material, and are normally contained in thecathode active material-containing layer 28.

There are no particular limitations on the current collector 16 and thecurrent collector 26 provided they are good conductors able toadequately transfer charge to the anode active material-containing layer18 and the cathode active material-containing layer 28, and knowncurrent collectors used in electrochemical devices can be used. Forexample, examples of the current collectors 16 and 26 include metalfoils each made of copper, aluminum and the like.

In addition, the anode active material-containing layer 18 of the anode10 is mainly composed of an anode active material and a binder.Furthermore, the anode active material-containing layer 18 preferablyalso contains a conductive auxiliary agent.

There are no particular limitations on the anode active materialprovided it allows occlusion and discharge of lithium ions, eliminationand insertion (intercalation) of lithium ions, or doping and dedoping oflithium ions and counter anions of said lithium ions (such as ClO₄ ⁻) toproceed reversibly, and known anode active materials can be used.Examples of such active materials include carbon materials such asnatural graphite, artificial graphite, non-graphitizable carbon, easilygraphitizable carbon or low temperature fired carbon, metals capable ofcompounding with lithium such as Al, Si or Sn, amorphous compoundsconsisting mainly of oxides such as SiO₂ or SnO₂, and lithium titaniumoxide (Li₄Ti₅O₁₂). Carbon materials are particularly preferable, andthose in which the interlayer distance d₀₀₂ of the carbon material is0.335 to 0.338 nm and the size Lc₀₀₂ of crystallites of the carbonmaterial is 30 to 120 nm are particularly preferable. Examples of carbonmaterials that satisfy these conditions include artificial graphite,meso carbon fibers (MCF) and meso carbon microbeads (MCMB). Furthermore,the interlayer distance d₀₀₂ and crystallite size Lc₀₀₂ can bedetermined by X-ray diffraction.

A known binder can be used for the binder used in the anode withoutlimitation, and examples of binders include fluororesins such aspolyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE),tetrafluoroethylene-hexafluoropropylene copolymer (FEP),tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA),ethylene-tetrafluoroethylene copolymer (ETFE),polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylenecopolymer (ECTFE) and polyvinyl fluoride (PVF). This binder not onlybinds constituent materials such as active material particles and aconductive auxiliary agent added as necessary, but also contributes toadhesion between these constituent materials and the current collectors.

In addition, other examples of binders that may be used includevinylidene fluoride-based fluorine rubbers such as vinylidenefluoride-hexafluoropropylene-based fluorine rubber VDF-HFP-basedfluorine rubber), vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene-based fluorine rubber(VDF-HFP-TFE-based fluorine rubber), vinylidenefluoride-pentafluoropropylene-based fluorine rubber (VDF-PFP-basedfluorine rubber), vinylidenefluoride-pentafluoropropylene-tetrafluoroethylene-based fluorine rubber(VDF-PFP-TFE-based fluorine rubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluorine rubber(VDF-PFMVE-TFE-based fluorine rubber) and vinylidenefluoride-chlorotrifluoroethylene-based fluorine rubber (VDF-CTFE-basedfluorine rubber).

Moreover, additional examples of binders that may be used includepolyethylene, polypropylene, polyethylene terephthalate, aromaticpolyamides, cellulose, styrene-butadiene rubber, isoprene rubber,butadiene rubber and ethylene-propylene rubber. In addition,thermoplastic elastomeric polymers such as styrene-butadiene-styreneblock copolymers and hydrogenation products thereof,styrene-ethylene-butadiene-styrene copolymers orstyrene-isoprene-styrene block copolymers and hydrogenation productsthereof may also be used. Moreover, syndiotactic 1,2-polybutadiene,ethylene-vinyl acetate copolymers or propylene-α-olefin (2 to 12 carbonatoms) copolymers and the like may also be used. In addition, conductivepolymers may also be used.

There are no particular limitations on the conductive auxiliary agentused as necessary, and known conductive auxiliary agents can be used,examples of which include carbon blacks, carbon materials, metal powderssuch as those of copper, nickel, stainless steel or iron, mixtures ofcarbon materials and metal powders, and conductive oxides such as ITO.

In addition, the cathode active material-containing layer 28 of thecathode 20 is mainly composed of a cathode active material and a binderin the same manner as the anode active material-containing layer 18. Inaddition, the cathode active material-containing layer 28 preferablyalso contains a conductive auxiliary agent. The cathode activematerial-containing layer 28 contains the composite particles for anelectrode of the present invention as a cathode active material.

Furthermore, the cathode active material-containing layer 28 may furthercontain a known cathode active material other than the compositeparticles for an electrode of the present invention. Examples of cathodeactive materials that can be used in combination therewith include) butare not limited to, those which allow occlusion and discharge of lithiumions, elimination and insertion (intercalation) of lithium ions, ordoping and dedoping of lithium ions and counter anions of said lithiumions (such as ClO₄ ⁻) to proceed reversibly, and known electrode activematerials can be used, specific examples of which include lithium cobaltoxide (LiCoO₂), lithium nickel dioxide (LiNiO₂), lithium manganesespinel (LiMn₂O₄) and composite metal oxides represented by the generalformula; LiNi_(x)Co_(y)Mn_(z)O₂ (wherein, x+y+z=1) as well as compositemetal oxides such as lithium vanadium compounds (LiV₂O₅), olivine typeLiMPO₄ (wherein, M represents Co, Ni, Mn or Fe) or lithium titaniumoxide (Li₄T₅O₁₂).

The same binders used for the anode 10 can be used for the cathode 20.In addition, the same conductive auxiliary agents used for the anode 10can be used for the cathode 20 as necessary.

In addition, the metal content in the cathode active material-containinglayer 28 based on the total mass of said cathode activematerial-containing layer 28 (content of metal supported on LiVOPO₄particles) is preferably 2 to 10% by mass, more preferably 2 to 8% bymass and particularly preferably 2 to 6% by mass. If this metal contentis less than 2% by mass, electron conductivity tends to be inadequate,while if the metal content exceeds 100% by mass, the amount of metal inthe electrode becomes excessively large and tends to lead to a decreasein electrode capacity due to a decrease in the proportion of activematerial.

Moreover, the content of the composite particles for an electrode of thepresent invention in the cathode active material-containing layer 28based on the total mass of said cathode active material-containing layer28 is preferably 80 to 97% by masse more preferably 85 to 95% by massand particularly preferably 90 to 95% by mass. If this content ofcomposite particles is less than 80% by mass, the electrical capacity asan electrode tends to decrease, while if the content of compositeparticles exceeds 97% by mass, the amount of metal and conductiveauxiliary agent in the electrode decreases and electron conductivitytends to decrease.

In addition, the current collector of the cathode 20 is electricallyconnected to one end of the cathode lead 22 composed of, for examplealuminum, while the other end of the cathode lead 22 extends outside thecase 50. On the other hand, the current collector of the anode 10 issimilarly electrically connected to one end of the anode lead 12composed of, for example, copper or nickel, while the other hand of theanode lead 12 extends outside the case 50.

There are no particular limitations on the separator 40 arranged betweenthe anode 10 and the cathode 20 provided it is formed from a porous bodyhaving ion permeability and is electrically insulating, and knownseparators used in electrochemical devices can be used. Examples of thisseparator 40 include film laminates composed of polyethylene,polypropylene or polyolefin, stretched films of mixtures of theabove-mentioned polymers, and fiber non-woven fabrics composed of atleast one type of constituent material selected from the groupconsisting of cellulose, polyester and polypropylene.

An electrolyte solution (not shown) is filled into a space within thecase 50, and a portion thereof is contained within the anode 10, cathode20 and separator 40. A non-aqueous electrolyte solution, in which alithium salt is dissolved in an organic solvent, is used for theelectrolyte solution. Examples of lithium salts used include LiPF₆,LiClO₄, LiBF₄, LiAsF₆, LiCF₉SO₃, LiCF₃CF₂SO₃, LiC(CF₃SO₂)₃,LiN(CF₃SO₂)₂, LiN(CF₃CF₂SO₂)₂, LiN(CF₃SO₂)(C₄F₉SO₂) and LiN(CF₃CF₂CO)₂.Furthermore, one type of these salts may be used alone or two or moretypes may be used in combination. In addition, the electrolyte solutionmay be in the form of a gel by adding a polymer and the like.

A known solvent used in electrochemical devices can be used for theorganic solvent, preferable examples of which include propylenecarbonate, ethylene carbonate and diethyl carbonate. These may be usedalone or two or more types may be mixed in an arbitrary ratio.

The case 50 is formed using a pair of mutually opposing films (firstfilm 51 and second film 52). Here, as shown in FIG. 3, the first Film 51and the second film 52 are connected in the present embodiment. Namely,in the present embodiment, the case 50 is formed by bending arectangular film composed of a single composite packaging film alongbending line X3-X3 shown in FIG. 3, and then overlapping one set ofopposing edges of the rectangular film (edge 51B of the first film 51and edge 52B of the second film 52 in the drawing) followed by using anadhesive or carrying out heat-sealing. Furthermore, reference symbol 51Ain FIGS. 2 and 3 and reference symbol 52A in FIG. 3 indicate thoseregions of the first film 51 and the second film 52, respectively, thatare not adhered or heat-sealed.

The first film 51 and the second film 52 respectively indicate theportions of the film having mutually opposing surfaces formed whenbending the single rectangular film in the manner described above. Here,in the present description, the respective edges of the first film 51and the second film 52 after being joined are referred to as “sealedportions.”

As a result, since it is no longer necessary to provide a sealed portionfor joining the first film 51 and the second film 52 at the portion ofthe bending line X3-X3, the sealed portion of the case 50 can bereduced. As a result, the volume energy density of the electrochemicaldevice 1 based on the volume of the space where it is to be installedcan be further improved.

In addition, in the case of the present embodiment, one end of the anodelead 12 connected to the anode 10, and one end of the cathode lead 22connected to the cathode 20 are respectively arranged so as to protrudeoutside the sealed portion where edge 51B of the first film 51 and edge52B of the second film 52 are joined as shown in FIGS. 2 and 3.

In addition, the film that composes the first film 51 and the secondfilm 52 is a flexible film. Since this film is lightweight and can beeasily formed into a thin film, it allows the form of theelectrochemical device itself to be thin. Consequently, in addition tobeing able to easily improve the inherent volume energy density, thevolume energy density based on the volume of the space where theelectrochemical device is to be installed can also be easily improved.

Although there are no particular limitations on this film provided it isa flexible film, from the viewpoint of effectively preventingpenetration of water and air from outside the case 50 to inside the case50 as well as the escape of electrolyte components from inside the case50 to outside the case 50 while ensuring adequate mechanical strengthand light weight of the case, the film is preferably a “compositepackaging film” at least having an innermost polymer layer in contactwith a power generation unit 60 and a metal layer arranged on theopposite side of the side of the innermost layer in contact with thepower generation unit.

Examples of composite packaging films able to be used for the first film51 and the second film 52 include the composite packaging films composedas shown in FIGS. 7 and 8. A composite packaging film 53 shown in FIG. 7has an innermost polymer layer 50 a in contact with the power generationunit 60 on an inner surface F53 thereof, and a metal layer 50 c arrangedon the other surface (outer side) of the innermost layer 50 a. Inaddition, a composite packaging film 54 shown in FIG. 8 employs aconfiguration in which a polymer outermost layer 50 b is furtherarranged on the outside of the metal layer 50 c of the compositepackaging film 53 shown in FIG. 7.

Although there are no particular limitations on the composite packagingfilm able to be used for the first film 51 and the second film 52provided it is a compound packaging material provided with two or morelayers consisting of one or more polymer layers, including the innermostlayer described above, and a metal layer such as a metal foil, from theviewpoint of reliably obtaining effects similar to those describedabove, it is preferably composed of three or more layers consisting ofthe innermost layer 50 a, the outermost polymer layer 50 b arranged onthe outer surface of the case 50 farthest from the innermost layer 50 a,and at least one metal layer 50 c arranged between the innermost layer50 a and the outermost layer 50 b as in the composite packaging film 54shown in FIG. 8.

The innermost layer 50 a is a flexible layer, and although there are noparticular limitations on the constituent material thereof provided itis a polymer capable of demonstrating flexibility as described above andhas chemical stability with respect to the non-aqueous electrolyte used(characteristics that prevent the occurrence of chemical reactions,dissolution and swelling) as well as chemical stability with respect tooxygen and water (water in the air), a material is preferable also hasthe characteristic of low permeability with respect to oxygen, water(water in the air) and components of the non-aqueous electrolytesolution, examples of which include engineering plastics andthermoplastic resins such as polyethylene, polypropylene, acid-modifiedpolyethylene, acid-modified polypropylene, polyethylene ionomers andpolypropylene ionomers.

Furthermore, “engineering plastics” refer to plastics having superiormechanical properties, heat resistance and durability used in mechanicalparts, electrical parts, housing materials and the like, and examplesinclude polyacetals, polyamides, polycarbonates, polyoxytetramethyleneoxyterephthaloyls (polybutylene terephthalates), polyethyleneterephthalates, polyimides and polyphenylene sulfides.

In addition, in the case of further providing a polymer layer such asoutermost layer 50 b in addition to the innermost layer 50 a as in thecomposite packaging film 54 shown in FIG. 8, the same constituentmaterials as those of the innermost layer 50 a may be used for thispolymer layer.

The metal layer 50 c is preferably a layer formed from a metal materialhaving corrosion resistance to oxygen, water (water in the air) andnon-aqueous electrolyte solutions, and examples of metal materials thatmay be used include metal foils made of aluminum, aluminum alloy,titanium or chromium and the like.

In addition, although there are no particular limitations on the methodused to seal all of the sealed portions in the case 50, heat sealing ispreferable from the viewpoint of productivity.

As shown in FIGS. 2 and 3, an insulator 14, for preventing contactbetween the anode lead 12 and the metal layer in the composite packagingfilm composing each film, is coated onto the portion of the anode lead12 contacting the package sealed portion composed of edge 51B of thefirst film 51 and edge 52B of the second film 52. Moreover, an insulator24, for preventing contact between the cathode lead 22 and the metallayer in the composite packaging film composing each film, is coatedonto the portion of the cathode lead 22 contacting the package sealedportion composed of edge 51B of the first film 51 and edge 52B of thesecond film 52.

Although there are no particular limitations on the configuration ofthese insulators 14 and 24, they each may be formed from a polymer.Furthermore, a configuration may also be employed in which theseinsulators 14 and 24 are not arranged provided respective contact withthe metal layer in the composite packaging film by the anode lead 12 andthe cathode lead 22 can be adequately prevented.

Next, the above-mentioned electrochemical device 1 can be fabricated by,for example, the procedure described below. First, the anode lead 12 andthe cathode lead 22 are electrically connected to the anode 10 and thecathode 20, respectively. Subsequently, the separator 40 is arranged incontact (but preferably not adhered) with the anode 10 and the cathode20 there between to complete the power generation unit 60.

Next, the case 50 is fabricated by, for example, the method describedbelow. First, in the case of composing the first film and the secondfilm from a composite packaging film as described above, the case isfabricated using a known production method such as dry lamination, wetlamination, hot melt lamination or extrusion lamination. In addition, afilm to serve as the polymer layer composing the composite packagingfilm, and a metal foil made of aluminum and the like are prepared. Themetal foil can be prepared by, for example, rolling a metal material.

Next, a composite packaging film (multilayer film) is preferablyfabricated by, for example, laminating the metal foil onto the filmserving as the polymer layer by means of an adhesive so as to form themultilayer configuration previously described. The composite packagingfilm is then cut to a predetermined size to prepare a single rectangularfilm.

Next, as previously explained with reference to FIG. 3, the single filmis bent and sealed portion 51B (edge 51B) of the first film 51 and thesealed portion 52B (edge 52B) of the second film 52 are, for example,heat-sealed over a desired sealing width under predetermined heatingconditions using a sealing machine. At this time, a portion is providedwhere a portion of the heat sealing is not carried out to secure anopening for introducing the power generation unit 60 into the case 50.As a result, the case 50 is obtained having an opening therein.

The power generation unit 60, to which the anode lead 12 and the cathodelead 22 are electrically connected, is inserted into the case 50 havingan opening. Electrolyte solution is then injected inside. Continuing,the opening of the case 50 is then sealed using a sealing machine with aportion of the anode lead 12 and the cathode lead 22 respectivelyinserted into the case 50. Fabrication of the case 50 and theelectrochemical device 1 is completed in this manner. Furthermore, anelectrochemical device of the present invention is not limited to thisform, but rather may also be in the form of a cylinder and the like.

Although the above has provided a detailed explanation of one preferredembodiment of an electrochemical device of the present invention, thepresent invention is not limited to the above-mentioned embodiment. Forexample, in the explanation of the embodiment as described above, theconfiguration may be made more compact by bending the sealed portion ofthe electrochemical device 1. In addition, although the explanation ofthe embodiment as described above explained an electrochemical device 1provided with one anode 10 and one cathode 20 each, a configuration mayalso be employed in which two or more of anode 10 and cathode 20 areprovided, and a single separator 40 is always arranged between the anode10 and the cathode 20.

In addition, although the explanation of the embodiment described aboveexplained the case of the electrochemical device being a lithium ionsecondary battery, for example, an electrochemical device of the presentinvention is not limited to a lithium ion secondary battery, but rathermay also be a secondary battery other than a lithium ion secondarybattery such as a lithium metal secondary battery (that which uses thecomposite particles of the present invention for the cathode and lithiummetal for the anode) or an electrochemical capacitor such as a lithiumcapacitor. In addition, an electrochemical device of the presentinvention can also be used in applications such as a power supply of aself-propelled micro-machine or IC card, or a distributed power supplyarranged on a printed circuit board or in a printed circuit board.Furthermore, in the case of an electrochemical device other than alithium ion secondary battery, an active material suitable for therespective electrochemical device may be used for the active materialother than the composite particles of the present invention.

EXAMPLE

Although the following provides a more detailed explanation of thepresent invention based on examples and comparative examples thereof,the present invention is not limited to the following examples.

Example 1

A Li source in the form of Li₂CO₃, a V source in the form of V₂O₅ and aPO₄ source in the form of NH₄H₂PO₄ were mixed at the stoichiometricratio of LiVOPO₄ followed by firing for 12 hours at 600° C. to obtainLiVOPO₄ particles (average particle diameter: 4.2 μm). The resultingLiVOPO₄ particles were dispersed in an aqueous solution in which wasdissolved HAuCl₄ at 0.01 molly to obtain an LiVOPO₄/Au precursorLiVOPO₄/Au composite particles were obtained in which a metal coatinglayer (Au coating layer) was formed on at least a portion of the surfaceof the LiVOPO₄ particles by removing the solvent from this precursor andheat-treating at 300° C.

The Au content of the resulting composite particles based on the totalmass of the composite particles was 3.2% by mass, and the thickness ofthe Au coating layer was 50 nm.

Next, 93 parts by mass of the resulting composite particles, 2 parts bymass of a conductive auxiliary agent in the form of acetylene black, and5 parts by mass of a binder in the form of polyvinylidene fluoride(PVDF) were mixed followed by preparing a slurry for forming an activematerial-containing layer by dispersing in N-methyl-2-pyrrolidone (NMP).This slur was coated onto a current collector in the form of an aluminumfoil and dried followed by rolling to obtain an electrode in which anactive material-containing layer having a thickness of 40 μm was formedon a current collector having a thickness of 20 μm. The carbon contentof the active material-containing layer based on the total mass of theactive material-containing layer was 2% by mass, while the Au contentwas 3% by mass.

Next, the resulting electrode and a counter electrode thereof in theform of a Li foil (thickness: 100 μm) were laminated with a separatorcomposed of a polyethylene microporous film interposed there between toobtain a laminate (element). This laminate was inserted into an aluminumlaminator pack followed by injecting an electrolyte in the form of a 1 MLiPF₆ solution (solvent: EC/DEC=3/7 (mass ratio)) into this aluminumlaminator pack and vacuum-sealing the pack to fabricate an evaluationcell (length: 48 mm, width: 34 mm, thickness: 2 mm).

A constant current discharge test was carried out at a dischargetemperature of 25° C. using this evaluation cell to measure dischargevoltage and discharge capacity at 1/20 C. as well as discharge voltageand discharge capacity at 1 C. Those results are shown in Table 1.

Comparative Example 1

LiVOPO₄ particles (average particle diameter 4.5 μm) were obtained inthe same manner as Example 1. 80 parts by mass of the resulting LiVOPO₄particles, 15 parts by mass of a conductive auxiliary agent in the formof acetylene black and 5 parts by mass of a binder in the form ofpolyvinylidene fluoride (PVDF) were mixed followed by preparing a slurryfor forming an active material-containing layer by dispersing inN-methyl-2-pyrrolidone (NMP). This slurry was coated onto a currentcollector in the form of an aluminum foil and dried followed by rollingto obtain an electrode in which an active material-containing layerhaving a thickness of 40 μm was formed on a current collector having athickness of 20 μm. The carbon content of the active material-containinglayer based on the total mass of the active material-containing layerwas 15% by mass.

An evaluation cell was fabricated in the same manner as Example 1 withthe exception of using this electrode. The resulting evaluation cell wasthen used to measure discharge voltage and discharge capacity at 1/20 Cas well as discharge voltage and discharge capacity at 1 C in the samemanner as Example 1. Those results are shown in Table 1.

Comparative Example 2

A Li source in the form of Li₂CO₃, an Fe source in the form of FeSO₄ anda PO₄ source in the form of NH₄H₂PO₄ were mixed at the stoichiometricratio of LiFePO₄ followed by firing for 12 hours at 600° C. withacetylene black to obtain LiFePO₄ particles (average particle diameter:3.9 μm).

93 parts by mass of the resulting LiFePO₄/carbon composite particles, 2parts by mass of a conductive auxiliary agent in the form of acetyleneblack, and 5 parts by mass of a binder in the form of polyvinylidenefluoride (PVDF) were mixed followed by preparing a slurry for forming anactive material-containing layer by dispersing in N-methyl-2-pyrrolidone(NMP). This slurry was coated onto a current collector in the form of analuminum foil and dried followed by rolling to obtain an electrode inwhich an active material-containing layer having a thickness of 40 μmwas formed on a current collector having a thickness of 20 μm. Thecarbon content of the active material-containing layer based on thetotal mass of the active material-containing layer was 6% by mass.

An evaluation cell was fabricated in the same manner as Example 1 withthe exception of using this electrode. The resulting evaluation cell wasthen used to measure discharge voltage and discharge capacity at 1/20 Cas well as discharge voltage and discharge capacity at 1 C in the samemanner as Example 1. Those results are shown in Table 1.

TABLE 1 Constant Current Constant Current Discharge at 1/20 C Dischargeat 1 C Discharge Discharge Discharge Discharge Coating Capacity VoltageCapacity Voltage Layer (mAh/g) (V) (mAh/g) (V) Example 1 Metal 116 3.8283 3.58 coating layer present Comparative Metal 88 3.84 30 3.60 Example1 coating layer absent Comparative Metal 130 3.35 82 2.80 Example 2coating layer absent

What is claimed is:
 1. Composite particles for an electrode comprising:LiVOPO₄ particles; and a metal, wherein: the metal is supported on atleast a portion of a surface of the LiVOPO₄ particles to form a metalcoating layer, the metal contains at least one type selected from thegroup consisting of Al, Au, and Pt, and when a length of an outercircumference of the LiVOPO₄ particles in a cross-section of thecomposite particles for an electrode is designated as L, and a length ofa portion of the outer circumference of the LiVOPO₄ particles where themetal coating layer is formed is designated as L′, then a coating ratiorepresented by (L′/L) is 0.2 or more.
 2. The composite particles for anelectrode according to claim 1, wherein a BET specific surface area is0.5 to 10.0 m²/g.
 3. The composite particles for an electrode accordingto claim 1, wherein the thickness of the metal coating layer is 10 to300 nm.
 4. An electrochemical device, comprising an electrode containingthe composite particles for an electrode according to claim
 1. 5. Thecomposite particles for an electrode according to claim 1, wherein acontent of metal supported on the LiVOPO₄ particles, based on a totalmass of the composite particle, is 2.0 to 5.0 by mass %.
 6. A processfor producing composite particles for an electrode which compriseLiVOPO₄ particles and a metal, the metal being supported on at least aportion of a surface of the LiVOPO₄ particles to form a metal coatinglayer, the process comprising: a fluidized layer formation step offorming the metal coating layer on at least the portion of the surfaceof the LiVOPO₄ particles by introducing the LiVOPO₄ particles and metalparticles into a fluidized bed in which an air flow has been generatedand forming a fluidized layer, wherein: the metal contains at least onetype selected from the group consisting of Al, Au, and Pt, and when alength of an outer circumference of the LiVOPO₄ particles in across-section of the composite particles for an electrode is designatedas L, and a length of a portion of the outer circumference of theLiVOPO₄ particles where the metal coating layer is formed is designatedas L′, then a coating ratio represented by (L′/L) is 0.2 or more.
 7. Aprocess for producing composite particles for an electrode whichcomprise LiVOPO₄ particles and a metal, the metal being supported on atleast a portion of a surface of the LiVOPO₄ particles to form a metalcoating layer, the process comprising: a dispersion step of introducingthe LiVOPO₄ particles into a metal ion-containing solution to obtain anLiVOPO₄ dispersion; and a reduction step of subjecting the LiVOPO₄dispersion to reduction treatment, wherein: the metal contains at leastone type selected from the group consisting of Al, Au, and Pt, and whena length of an outer circumference of the LiVOPO₄ particles in across-section of the composite particles for an electrode is designatedas L, and a length of a portion of the outer circumference of theLiVOPO₄ particles where the metal coating layer is formed is designatedas L′, then a coating ratio represented by (L′/L) is 0.2 or more.